1. Technical Field
The present invention relates to a disk drive, a spindle motor, and a bearing mechanism for the spindle motor. In particular, the present invention relates to suppressing changes in the pre-load pressure of a bearing as a result of temperature change of the bearing.
2. Description of the Related Art
Referring to FIG. 7, a sectional view of a conventional spindle motor used in a hard disk drive is shown. A cylindrical shaft 102 made of stainless steel is mounted in the center of a bottom part 101 covering a bottom surface part of a spindle motor 100. Shaft 102 fixedly holds inner races 103a and 104a, which are included in a pair of bearings 103 and 104, respectively. A space is left in the direction of a central axis 102X of shaft 102. Outer races 103b and 104b of bearings 103 and 104, respectively, are fixedly held by outer race holding members 106 and 107, whose inner diameters are formed slightly larger than one of an inner circumferential surface 105a. A space is left in the inner circumferential surface 105a of a hub 105.
An annular concave part 108 is formed in the hub 105, made of stainless steel, and faces the bottom part 101. Rotor magnets 109 are disposed on the inner circumferential surface 108a. A cylindrical central wall part 101a, which projects upward so as to support the shaft 102, is formed in a central part of the bottom part 101. On an outer circumferential surface of central wall part 101a, a selected number of core members 110, where stator coils 111 are wound, are fixedly disposed at equal intervals in the circumferential direction with an end part of each core member 110 facing each rotor magnet 109. An outer circumferential surface 105b of the hub 105 has a predetermined outer diameter and length in the axial direction such that the outer circumferential surface 105b fits within center holes of a selected number of disks (not shown).
If bearing steel is used to form both the balls and the inner and outer races of the bearing, the conventional bearing mechanism of the spindle motor that is described above will experience several hardness problems. For example, when the bearing is used at high rotational speeds (e.g., 10,000 rpm), the durability and bearing life are diminished due to lack of hardness.
In a contact-start-stop hard disk drive, disk vibration is suppressed because the sliders are in contact with the disk surfaces when the disks are not rotating. However, in a hard disk drive having a load/unload mechanism such as that shown in FIG. 1 (described below), disk vibration is not suppressed since the actuator arm is not in contact with the hub, especially when unloading.
Therefore, if steel balls are used in a bearing of a spindle motor in a hard disk drive having a load/unload mechanism, fluctuations with small amplitude occur between the inner and outer races and steel balls of the bearing when the disk drive is transported, etc. Consequently, fretting occurs in the contacting parts. Since the contacting surfaces become unlubricated because of extrusion of lubricant from the contacting surfaces, the contacting surfaces generate reddish brown abrasion powder, are eventually worn out, and concave surfaces are formed in them.
If ceramic balls are used in the bearing of the conventional spindle motor described above, some of these problems are alleviated. Unfortunately, a new problem occurs as follows. A ceramic ball has a coefficient of linear expansion that is smaller than that of the other bearing members formed from bearing steel. For example, even if a mechanism is designed so that an optimum pre-loaded pressure may be applied at room temperature, size ratios of the ceramic ball to other members decrease as temperature rises, and hence the pre-loaded pressure decreases. Hence, it is conceivable to set the pre-loaded pressure at room temperature to be higher than the optimum value supposing the use of the bearing at high temperature. Unfortunately, the bearing cannot be used at or below room temperature.
Thus, an object of the present invention is to provide a ceramic ball bearing mechanism that maintains stable operation, in spite of temperature changes, by suppressing changes in pre-load pressure that are caused by the temperature change.
A bearing mechanism of the present invention has a first bearing and a second bearing disposed at different positions in an axial direction. Each bearing has an inner race, an outer race, and rolling balls. A shaft holds the inner races of both bearings in an axially separated manner, while a supporting member holds the outer races of both bearings in an axially separated manner. The rolling balls of the bearings have a different coefficient of linear expansion than that of the inner and outer races. The inner and outer races of each of the bearings are fixed in an axially shifted manner, whereby pre-loaded pressures are applied between both the inner race and the balls, and between both the outer race and the balls. As the temperature changes, the supporting member has a compensation member that can change a distance between the outer races in a changing ratio different from a changing ratio of the distance between the inner races.
In one version, the inner races and the outer races are made of bearing steel, and the balls are ceramic. In another version, the shaft is made of stainless steel and the compensation member is made of aluminum.
A spindle motor of another form of the present invention has the bearing mechanism described above, and a bottom part fixedly supporting the shaft. The rotor magnets are held by the supporting member and are disposed along a circumference whose center is a central axis of the shaft. The cores are fixedly disposed on the shaft and have stator coils wound around the cores so that end parts of the coils may face the rotor magnets, respectively.
A spindle motor of still another form of the present invention has rotor magnets that are held by holding means, which is formed on the shaft in one piece, and are disposed along a circumference whose center is a central axis of the shaft. The cores are fixedly disposed on the shaft and have stator coils wound around the cores so that end parts of the cores may face the rotor magnets, respectively.
A disk drive of a further form of the present invention has the spindle motor described above, a disk that is held by a rotary part of this spindle motor and rotates in one piece, and an actuator arm holding a head scanning a recording surface of the disk.